1. Field of the Invention
The present invention relates to a method of manufacturing super-absorbent polymer (SAP), particularly a method of manufacturing super-absorbent polymer (SAP) which is powdery, insoluble in water, and able to absorb water, blood and urine with slight soluble substances.
2. Description of the Related Prior Art
Super-absorbent polymer (SAP) is of great water holding capacity, which is able to absorb water of hundred- and even thousand-fold of its weight, and is swelled after absorption so that water is kept in immobile state and from leaking out, even under pressure; by the way, the absorbed water is released to atmosphere slowly. Due to having the above-mentioned characteristics, SAP was used as water holding agent for soil in forest and agriculturally at early date. Recently, it is widely used in sanitary products, such as diapers, adult incontinence products, and feminine sanitary products as absorbent, and in applications for preserving foods, owing to the progressing of method of manufacturing SAP.
The constituent materials of SAP comprise hydrolyzed starch-acrylonitrile grafted polymers (JP-Kokoku-49(1974)-43,395);neutralized starch-acrylic acid grafted polymers (JP-Kokoku-51(1976)-125,468); saponified propenyl vinylacetate co-polymers (JP-Kokoku-52(1977)-14,689); hydrolyzed acrylonitrile polymers or acrylamide co-polymers (JP-Kokoku-53(1978)-15,959); partially neutralized polyacrylic acids (JP-Kokoku-55(1980)-84,304), etc. Among them, the SAP obtained by the cross-linking reactions between acrylic acids and acrylates are most common and economical, because acrylic acids are readily commercial available, the manufactured SAP are of excellent absorption ability, and the manufacturing processes are low cost with economical benefits.
The absorbent polymer is an insoluble hydrophilic polymer. Generally, polymer is of homogeneous bridging structure internally. A further bridging is conducted on surface of polymer after screening for given particle diameter in order to improve the properties, for example, to increase absorption rate, gel strength, blocking resistance, and liquid permeability. Such surface cross-linking is conducted by means of multifunctional cross-linking agents capable of reacting with acidic groups, as disclosed in many patent literatures, for example, the surface cross-linking is done with the dispersive absorbent polymers and the cross-linking agents conducted in organic solvent (JP-Kokoku-56(1981)-131608, JP-Kokoku-57(1982)-44627, JP-Kokoku-58(1983)-42602, and JP-Laid-open-58(1983)-117222); treating with mixing cross-linking agents and cross-linking agent solutions in the form of inorganic powders into SAP (JP-Kokoku-60(1985)-163956, JP-Kokoku-60(1985)-255814); treating with vaporization after addition of cross-linking agents (JP-A-1(1989)-113406); surface cross-linking with organic solvent, water and polyatomic alcohol (JP-Kokoku-63(1988)-270741, JP-Kokoku-64(1989)-50707, JP-Kokoku-1(1989)-292004); processing with solvent, water and ether (JP-Laid-open-2-153903), and the like. Though these processes increase the absorption rate of polymer (resin) or the magnification of absorbing water under pressure, yet they tend to excessively decrease holding capacity and consequently lower the performance on practical uses.
Methods of manufacturing SAP with polyacrylic acids and acrylates have been well developed and applied in the industry. The polymerization processes can be mentioned as followings, such as: cast film polymerization (JP-Kokoku-48(1973)-42,466); polymerization conducted on conveyer belt (JP-Kokoku-58(1983)-49,714); polymerization conducted in kneader with stirring blades for comminuting (JP-Kokoku-57(1982)-34,101); reserve phase suspension polymerization (JP-Kokoku-59(1984)-37,003); or polymerization conducted by spraying or coating monomers on fiber matrix (JP-Kokoku-62(1987)-53,309).
However, the above-mentioned conventionally polymerization processes are basically directed to the radical polymerization of monomers, polymerization initiators and cross-linking agents to form SAP as gel-like solid, so that it is difficult to control the reaction of polymerization and thus, the soluble substances will be higher.